Sheathed resistance heater

ABSTRACT

This invention relates to a sheathed resistance heater which comprises a heating wire (2) received in a metallic pipe 3 and electrically insulating powder (4) packed in the pipe. A metallic powder of the specific type is added to the electrically insulating powder (4), by which the heating wire (2) has a prolonged life by suppressing metal components in the wire from evaporating, the heater being widely applicable for domestic and industrial purposes.

TECHNICAL FIELD

This invention relates to a sheathed resistance heater of the type whichcomprises a heating wire received in a metal pipe and an electricallyinsulating powder packed in the metal pipe by which the resistanceheater has a prolonged life and can be maintained at a high level ofinsulation resistance in a working condition after having been used overa long term.

BACKGROUND ART

Sheathed or shielded resistance heaters have widely been used in manyfields as heating parts because of their very excellent performance,quality and convenience. The commercial range of the heaters has nowincreased including not only domestic electric articles, but alsospecific applications such as in various industries, space developmentsand atomic power services. Among various classes of the sheathedelectric heaters, sheathed heaters for high temperature purposes willmore and more increase in applications.

Upon reviewing the performance and quality of sheathed resistanceheaters in a world-wise sense, it will be found that they have defectsin that the insulation resistance in a working condition (hereinafterreferred to as insulation resistance under self-heating conditions)lowers as a function of time, coupled with another disadvantage in thatit takes only a short time before breakage of the heating wire.

DISCLOSURE OF THE INVENTION

The present invention contemplates to provide a sheathed resistanceheater in which there is used an electrically isnulating powder whichcontains a specific type of a powder therein whereby the life beforebreakage of the heating wire is prolonged and the heater exhibits a highinsulation resistance when measured under working or self-heatingconditions after long-term use.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a conventional sheathed resistance heater;and

FIGS. 2 through 13 refer to embodiments of the present invention, inwhich FIGS. 2, 5, 8 and 11 are, respectively, characteristic graphsshowing the relation between the total test period and the insulationresistance under self-heating conditions, FIGS. 3, 6, 9 and 12 arecharacteristic graphs showing the relation between the amount ofmetallic powder and the insulation resistance under self-heatingconditions; and FIGS. 4, 7, 10 and 13 are characteristic graphs showingthe relation between the amount of metallic powder and the life.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention are described with reference to theaccompanying drawings.

In general, a sheathed resistance heat comprises, as particularly shownin FIG. 1, a coil-like heating wire 2 provided with terminal bars 1 atopposite ends thereof, a metal pipe 3 receiving the wire therein, anelectrically insulating powder 4 such as electrofused magnesia,electrofused silica, electrofused alumina and the like filled up in themetal pipe 3, and optionally, a glass 5 and a heat-resistant resin 6sealing opposite ends of the metal pipe 3 therewith.

We have paid particular attention to the electrically insulating powder4 and made extensive studies on the powder.

EXAMPLE 1

An electrofused magnesia powder was used as a main component of theelectrically insulating powder 4, to which were added different amountsof nickel powder and mixed together to obtained samples of electricallyinsulating powder 4.

The electrofused magnesia powder used had a composition indicated inTable 1.

                  TABLE 1                                                         ______________________________________                                        MgO         96-97 wt %                                                        CaO         0.2-0.3 wt %                                                      SiO.sub.2   2-3 wt %                                                          Al.sub.2 O.sub.3                                                                          0.4-0.5 wt %                                                      Fe.sub.2 O.sub.3                                                                          0.14-0.16 wt %                                                    ______________________________________                                    

The heating wire 2 used was a nichrome wire of the first kind having adiameter of 0.29 mm in the form of a coil having a winding diameter of 2mm. The heating wire was connected with terminal bars 1 at opposite endsthereof.

The metal pipe 3 was a NCF 2P pipe (commercial name Incoroi 800) havinga length of 413 mm, an outer diameter of 8 mm and a thickness of 0.46mm. Into the metal pipe 3 was inserted the heating wire 2 connecting theterminal bars 1 at opposite ends thereof. Subsequently, the electricallyinsulating powder 4 which had been previously prepared was charged intothe metal pipe 3, followed by subjecting the metal pipe 3 to steps ofrolling for reduction of the diameter and annealing (1050° C., 10minutes). The resulting metal pipe 3 had a length of 500 mm and an outerdiameter of 6.6 mm. The metal pipe 3 was then sealed at opposite endswith a low melting glass 5 and a heat-resistant resin 6. Thus, sheathedresistance heaters of sample Nos. 12-17 were obtained.

It will be noted that the amount of nickel powder in the insulatingpowders for use in the sheathed heaters of sample Nos. 12-17 are shownin Table 2.

For comparison purposes, a conventional sheathed heater (sample No. 11)was made using, as the insulating powder 4, an electrofused magnesiapowder alone having the composition indicated in Table 1.

Each of the sheathed heaters of sample Nos. 11-17 was tested in thefollowing manner to determine its insulation and life performances.

As an initial characteristic of each of the finished samples, there wasmeasured an insulation resistance under conditions where the metal pipewas heated up to a surface temperature of 750° C. The results are shownin Table 2.

The heating wire 2 of each sample was continuously energized so that thesurface temperature of the metal pipe 3 was maintained at 750° C. todetermine a variation of insulation resistance under self-heatingconditions. Upon measurement of the insulation resistance underself-heating conditions, the surface temperature of the metal pipe 3 waslowered down to 750° C. The variation of the insulation resistance isshown in FIG. 2. In FIG. 2, curves 11-17 show variations of theinsulation resistance under self-heating conditions of the respectivesheathed heaters of sample Nos. 11-17.

In Table 2, there are shown the values of the insulation resistancemeasured 11 days after commencement of the continuous energizing test.FIG. 3 shows the relation between the amount of nickel powder and theinsulation resistance value under self-heating conditions 11 days aftercommencement of the continuous energization.

Moreover, the respective sheathed heaters of sample Nos. 11-17 werecontinuously energized until the heating wire was broken to determinethe number of days (life) before occurrence of the breakage. The resultsare shown in Table 2 and the relation between the amount of nickelpowder and the life is shown in FIG. 4.

                  TABLE 2                                                         ______________________________________                                                                    Insulation                                             Amount   Initial Insulation                                                                          Resistance under                                  Sam- of Nickel                                                                              Resistance under                                                                            Self-heating                                      ple  Powder   self-heating Condi-                                                                         Conditions Life                                   No.  (wt %)   tions (Mega Ohms)                                                                           (Mega Ohms)                                                                              (Days)                                 ______________________________________                                        11   0.0      20             0.95      13                                     12   0.1      14            1.1        16                                     13   0.3      11            1.3        20                                     14   1.0      6             1.4        35                                     15   3.0      0.85          1.3        46                                     16   10.0     0.02          1.1        20                                     17   30.0     <0.01         --          9                                     ______________________________________                                    

As will be apparent from Table 2 and FIG. 2, the sheathed heaters ofsample Nos. 12-16 in which the amount of the nickel powder in theinsulating powder is in the range of 0.1-10 wt% had almost the samelevel of the insulation resistance under self-heating conditions afterlong-term use as the known sheathed heater of sample No. 11. Thesheathed heater of sample No. 17 was found to be so low in theinsulation resistance that it could not stand practical use.

FIG. 3 reveals that the sheathed heaters in which the content of thenickel powder is in the range of 0.1-10 wt% have almost the sameinsulation resistance under self-heating conditions 11 days aftercommencement of the continuous energizing test as the sheathed heater ofsample No. 11.

FIG. 4 reveals that the sheathed heaters in which the content of thenickel powder is in the range of 0.1-10 wt% are longer in life than theknown sheathed heater of sample No. 11.

Thus, the sheathed resistance heaters using electrically insulatingpowders 4 which had a content of nickel powder ranges from 0.1-10 wt%were found to have a relatively high level of insulation resistanceunder self-heating conditions after long-term use and a prolonged life.

EXAMPLE 2

An electrofused magnesia powder was used as a main component of theelectrically insulating powder 4, to which were added different amountsof an electrofused cobalt powder, followed by mixing to obtain severalsamples of electrically insulating powders 4.

The procedure of Example 1 was subsequently repeated to fabricatesheathed resistance heaters of sample Nos. 18-23.

These sheathed resistance heaters were subjected to the measurement ofthe initial insulation resistance under self-heating conditions,variation in insulation resistance under self-heating conditions inrelation to time, and life.

Among the results of the measurement, the initial insulation resistanceunder self-heating conditions, insulation resistance under self-heatingconditions after 11 days and life are shown in Table 3.

FIG. 5 shows the insulation resistance under self-heating conditions inrelation to time, FIG. 6 shows the relation between the amount of cobaltpowder and the insulation resistance under self-heating conditions after11 days, and FIG. 7 shows the relation between the amount of the cobaltpowder and the life.

It will be noted that curves 18-23 in FIG. 5 correspond sheathed heatersof sample Nos. 18-23, respectively.

                  TABLE 3                                                         ______________________________________                                                      Initial Insulation                                                                         Insulation Resis-                                       Amount   Resistance under                                                                           tance under Self-                                  Sam- of Cobalt                                                                              Self-heating heating Conditions                                 ple  Powder   Conditions   after 11 Days                                                                             Life                                   No.  (wt %)   (Mega Ohms)  (Mega Ohms) (days)                                 ______________________________________                                        11   0.0      20           0.95        13                                     18   0.1      12           1.1         16                                     19   0.3      9            1.2         21                                     20   1.0      7.8          1.4         42                                     21   3.0      2.1          1.4         50                                     22   10.0     0.04         1.2         22                                     23   30.0     <0.01        0.28        12                                     ______________________________________                                    

As will be apparent from the results of Table 3 and FIG. 5, the sheathedheaters of sample Nos. 18-22 in which the content of cobalt powder is inthe range of 0.1-10 wt% had almost the same level of insulationresistance as the known sheathed heater No. 11. The sheathed heater ofsample No. 23 is so low in insulation resistance under self-heatingconditions that it cannot stand practical use.

As is clearly seen from FIG. 6, the sheathed heaters which made use ofthe cobalt powder in amounts ranging from 0.1-10 wt% had insulationresistance values, as measured under self-heating conditions 11 daysafter commencement of the continuous energizing test, similar to that ofthe known sheathed heater No. 11.

Moreover, FIG. 7 reveals that the sheathed heaters in which the contentof cobalt powder in the insulating powder ranges from 0.1-10 wt% had alonger life than the known sheathed heater of sample No. 11.

Thus, the sheathed heaters using the electrically insulating powders 4having a cobalt powder content of 0.1-10 wt% did not lower in theinsulation resistance under self-heating conditions after long-term useand had a prolonged life.

EXAMPLE 3

An electrofused magnesia powder was used as a main component of theelectrically insulating powder 4 and admixed with different amounts ofiron powder to obtain samples of electrically insulating powders 4.

The general procedure of example 1 was repeated to fabricate sheathedresistance heaters numbered as 23-29.

These sheathed heaters were each subjected, in the same manner as inExample 1, to the measurement of the initial insulation resistance underself-heating conditions, variation of the insulation resistance underself-heating conditions in relation to time, and life.

Some of these results including the initial insulation resistance valuesunder self-heating conditions, insulation resistance values underself-heating conditions after 11 days, and life are shown in Table 4.

FIG. 8 shows the variation of the insulation resistance underself-heating conditions in relation to time, FIG. 9 shows the relationbetween the content of iron powder and the insulation resistance underself-heating conditions after 11 days, and FIG. 10 shows the relationbetween the content of iron powder and the life of sheathed heater.

In FIG. 8, curves 24-29 correspond to the respective sheathed heaters ofsample Nos. 24-29.

                  TABLE 4                                                         ______________________________________                                                      Initial Insulation                                                                         Insulation Resis-                                       Amount   Resistance under                                                                           tance under Self-                                  Sam- of Iron  Self-heating heating Conditions                                 ple  Powder   Conditions   after 11 Days                                                                             Life                                   No.  (wt %)   (Mega Ohms)  (Mega Ohms) (Days)                                 ______________________________________                                        11   0.0      20           0.95        13                                     24   0.1      13           1.0         16                                     25   0.3      7.5          1.1         24                                     26   1.0      5.4          1.2         48                                     27   3.0      1.1          1.2         55                                     28   10.0     0.36         1.1         24                                     29   30.0     <0.01        0.48        11                                     ______________________________________                                    

As is clearly seen from Table 4 and FIG. 8, the sheathed heaters ofsample Nos. 24-28 which make use of the insulating powders having aniron powder content ranging from 0.1-10 wt% had almost the sameinsulation resistance values as the known sheathed heater No. 11. Thesheathed heater of sample No. 29 was so low in the insulation resistanceunder self-heating conditions that it could not be served for practicalapplications.

FIG. 9 reveals that with the sheathed heaters in which the content ofiron powder was in the range of 0.1-10 wt%, the insulation resistancevalues under self-heating conditions 11 days after commencement of thecontinuous energizing test were almost the same as that of the sheathedheater of sample No. 11.

FIG. 10 reveals that the sheathed heaters in which the content of ironpowder in the insulating powders ranges from 0.1-10 wt% were longer inlife than the known sheathed heater numbered as 11.

Thus, the sheathed heaters making use of electrically insulating powders4 in which the content of iron powder ranges from 0.1-10 wt% did notlower in the insulation resistance as measured under self-heatingconditions after long-term use and had a prolonged life.

EXAMPLE 4

An electrofused magnesia powder was used as a main component of theelectrically insulating powder 4 and admixed with different amounts ofnickel and cobalt powders to obtain samples of electrically insulatingpowders 4. The nickel and cobalt powders were used in equal amounts.

Subsequently, the general procedure of Example 1 was repeated tofabricate sheathed resistance heaters of sample Nos. 30-35.

Each of these heaters was subjected to the measurement of the initialinsulation resistance under self-heating conditions, variation of theinsulation resistance under self-heating conditions in relation to time,and life in the same manner as in Example 1.

Some of these results are shown in Table 5 including the initialinsulation resistance values under self-heating conditions, insulationresistance values under self-heating conditions after 11 days of thecontinuous energizing test, and life.

FIG. 11 shows the variation of the insulation resistance underself-heating conditions in relation to time. FIG. 12 shows the relationbetween the total amount of the nickel and cobalt powders and theinsulation resistance under self-heating conditions after 11 days. InFIG. 13, there is shown the relation between the total amount of thecobalt and nickel powders and the life.

In FIG. 11, curves 30-35 correspond to the sheathed heaters of sampleNos. 30-35, respectively.

                  TABLE 5                                                         ______________________________________                                             Total                                                                         Amount   Initial Insulation                                                                         Insulation Resis-                                       of Nickel                                                                              Resistance under                                                                           tance under Self-                                  Sam- & Cobalt Self-heating heating Conditions                                 ple  Powders  Conditions   after 11 Days                                                                             Life                                   No.  (wt %)   (Mega Ohms)  (Mega Ohms) (Days)                                 ______________________________________                                        11   0.0      20           0.95        13                                     30   0.1      12           1.1         16                                     31   0.3      8.8          1.2         21                                     32   1.0      7.9          1.4         41                                     33   3.0      2.0          1.4         50                                     34   10.0     0.04         1.2         21                                     35   30.0     <0.01        0.28        12                                     ______________________________________                                    

As will be clearly seen from table 5 and FIG. 11, the sheathed heatersnumbered as 30-34 in which the total amount of the nickel and cobaltpowders are in the range of 0.1-10 wt% had almost the same insulationresistance as the known sheathed heater of sample No. 11. The sheathedheater of sample No. 35 was so low in insulation resistance underself-heating conditions that it could not be used.

As will be clearly seen from FIG. 12, the sheathed heaters in which thenickel and cobalt powders were used in total amounts ranging from 0.1-10wt% had almost the same level of the insulation resistance, as measuredunder self-heating conditions 11 days after commencement of thecontinuous energizing test, as the known sheated heater No. 11.

FIG. 13 reveals that the sheathed heaters in which there were usednickel and cobalt powders in total amounts ranging from 0.1-10 wt%showed a longer life than the known sheathed heater of sample No. 11.

As will be understood from the above results, the sheathed heatersmaking use of electrically insulating powders 4 which had a total amountof cobalt and nickel powders of 0.1-10 wt% did not lower in theinsulation resistance as measured under self-heating conditions afterlong-term use and had a prolonged life.

In the above examples, nickel, cobalt and iron were used as a metallicpowder being added and similar results were obtained when niobium,tungsten and yttrium are used instead of the above-mentioned metals.

In Examples 1-4, the electrofused magnesia powder was used as a maincomponent of the electrically insulating powder, and a similar tendencywas shown when electrofused alumina and silica powders were used insteadof the electrofused magnesia powder.

The characteristics of the sheathed heater may, more or less, varydepending on the kind of the electrofused magnesia powder. For instance,use of an electrofused magnesia powder having a high specific resistanceresults in a higher insulation resistance of sheathed heater and use ofan electrofused magnesia powder of high purity showing a relatively longlife results in a longer life of sheathed heater.

Although the nichrome wire of the first kind was used as the heatingwire 2, other wire materials indicated in Table 6 may be likewise usedwith similar results. As regards the metallic pipe 3, use of othermetals or alloys indicated in Table 7 produces similar results.

                                      TABLE 6                                     __________________________________________________________________________              Chemical Composition (%)                                            Kind      Ni  Cr  C   Si   Mn  Fe   Al                                        __________________________________________________________________________    Nichrome                                                                            First                                                                             over                                                                              19-21                                                                             below                                                                             0.75-1.5                                                                           below                                                                             below                                                                              --                                        Wire  Kind                                                                              77      0.15     2.5 1                                                    Second                                                                            over                                                                              15-18                                                                             below                                                                             0.75-1.5                                                                           below                                                                             balance                                                                            --                                              Kind                                                                              57      0.15     1.5                                                Iron- First                                                                             --  23-26                                                                             below                                                                             below                                                                              below                                                                             balance                                                                            4-6                                       Chromium-                                                                           Kind        0.10                                                                              1.5  1.0                                                Aluminium                                                                           Second                                                                            --  17-21                                                                             below                                                                             below                                                                              below                                                                             balance                                                                            2-4                                       Wire  Kind        0.10                                                                              1.5  1.0                                                __________________________________________________________________________

                                      TABLE 7                                     __________________________________________________________________________                Chemical Composition (%)                                          Kind        C   Si  Mn  P   S   Ni  Cr  Fe   Cu  Al                           __________________________________________________________________________    Stain-                                                                              SUS304                                                                              below                                                                             below                                                                             below                                                                             below                                                                             below                                                                              8.00˜                                                                      18.00˜                                                                      balance                                                                            --  --                           less        0.08                                                                              1.00                                                                              2.00                                                                              0.04                                                                              0.03                                                                              10.50                                                                             20.00                                     Steel SUS321                                                                              below                                                                             below                                                                             below                                                                             below                                                                             below                                                                              9.00˜                                                                      17.00˜                                                                      balance                                                                            --  --                                       0.08                                                                              1.00                                                                              2.00                                                                              0.04                                                                              0.03                                                                              13.00                                                                             19.00                                           SUS316L                                                                             below                                                                             below                                                                             below                                                                             below                                                                             below                                                                             12.00˜                                                                      16.00˜                                                                      balance                                                                            --  --                                       0.03                                                                              1.00                                                                              2.00                                                                              0.04                                                                              0.03                                                                              15.00                                                                             18.00                                     Corrosion                                                                           NCF1P below                                                                             below                                                                             below                                                                             below                                                                             below                                                                             over                                                                              14.00˜                                                                      6.00˜                                                                        below                            and Heat                                                                            (Inconery                                                                           0.15                                                                              0.50                                                                              1.00                                                                               0.030                                                                             0.015                                                                            72.00                                                                             17.00                                                                             10.00                                                                              0.50                             Resistant                                                                           600)                                                                    Super NCF2P below                                                                             below                                                                             below                                                                             below                                                                             below                                                                             30.00˜                                                                      19.00˜                                                                      balance                                                                            below                                                                             0.15                         Alloys                                                                              (Incoroi                                                                            0.10                                                                              1.00                                                                              1.50                                                                               0.030                                                                             0.015                                                                            35.00                                                                             23.00    0.75                                                                              0.60                               800)                                                                    __________________________________________________________________________

In Examples 1-4, the heaters were tightly sealed with the low meltingglass 5 and the heat-resistant resin 6 but a similar tendency was showneven though the heaters were not sealed.

The sheathed resistance heat of the present invention is not limited tothe design shown in FIG. 1 and may include those called a cartridgeheater and a glow plug.

INDUSTRIAL UTILIZABILITY

As described hereinabove, according to the present invention, there canbe obtained a sheathed resistance heater of a long life by using anelectrically insulating powder admixed with 0.1-10 wt% of at least onemetallic powder selected from the group consisting of those of nickel,cobalt, iron, niobium, tungsten and yttrium.

We claim:
 1. A sheathed resistance heater comprising an electrical metalheating wire received in a metal pipe, and an electrically insulatingpowder composition filling the remainder of the metal pipe, saidinsulating power composition comprising at least one electricallyinsulating powder uniformly mixed with at least one metallic powderselected from the group consisting of nickel, cobalt, iron, niobium,tungsten and yttrium, so that said heating wire is insulated from saidmetal pipe by said insulating powder composition and said at least onemetallic powder suppresses the evaporation of metal components in theheating wire.
 2. A sheathed resistance heater according to claim 1,wherein said at least one metal powder is added in an amount of 0.1-10wt%.
 3. A sheathed resistance heater comprising an electrical metalheating wire received in a metal pipe, and an electrically insulatingpowder composition filling the remainder of the metal pipe, saidinsulating powder composition comprising a uniform mixture of anelectrically insulating powder containing about 0.1 to 10 weight percentof at least one metallic powder additive selected from the groupconsisting of nickel, cobalt, iron, niobium, tungsten and yttrium, sothat said heating wire is insulated from said metal pipe by saidinsulating powder composition and said at least one metallic powdersuppresses the evaporation of metal components in the heating wire.
 4. Asheathed resistance heater according to claim 3, wherein saidelectrically insulating powder is magnesia powder.
 5. A sheathedresistance heater according to claim 3, wherein said metal pipe hassealed ends.